In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Paper making devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Paper making and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In conventional practice, paper making machines have several control stages with numerous, independently-controllable actuators that extend across the width of the sheet at each control stage. For example, a paper making machine will typically include a head box having a plurality of slices at the front which allow the stock in the head box to flow out on the fabric of the web or wire. The paper making machine might also include a steam box having numerous steam actuators that control the amount of heat applied to several zones across the sheet. Similarly, in a calendaring stage, a segmented calendaring roller can have several actuators for controlling the nip pressure applied between the rollers at various zones across the sheet.
All of the actuators in a stage are operated to maintain a uniform and high quality finished product. Such control might be attempted, for instance, by an operator who periodically monitors sensor readings and then manually adjusts each of the actuators until the desired output readings are produced. Paper making machines include control systems for automatically adjusting cross-directional actuators using signals sent from scanning sensors.
On-line measurements of sheet properties can be made in both the machine direction and in the cross direction. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
Cross-directional measurements are typically made using a scanning sensor that periodically traverses back and forth across the width of the sheet material. Current technology in paper making uses a beta type sensor that scans across the sheet during the manufacturing process, to measure basis weight. The objective of scanning across the sheet is to measure the variability of the sheet in both CD and MD. Based on the measurements, corrections to the process are made to make the sheet more uniform. A difficulty with this measurement technique is that while the sensor scans across 30 to 40 feet of the sheet, CD, 1000 to 2000 feet of paper have passed the sensor in the MD. This means that MD and CD information are mixed together during a scan. Further, the scanning sensor is capable of measuring only a small fraction of the paper produced. Another disadvantage is that the sheet shrinks as it dries, so corrections must be made to determine which actuator at the head box will affect the location being measured.
To separate CD information from the mix, it is typical to filter the data from several scans to average out MD variations. With filtering, it takes several minutes to obtain an accurate CD profile. The MD information is usually extracted by using the average of all readings across the sheet, i.e., "scan average." While these methods have proven reliable and accurate over the years, the main disadvantage is that they are slow and only less than 0.5% of the sheet is actually measured.
As is apparent, there is a need in the art for a system to obtain an accurate, high resolution basis weight measurement for a paper machine, especially one in which the CD profiles are obtained substantially instantaneously, and in which the MD and CD variations are substantially completely decoupled.